Environmental pollution caused by industrial and automotive by- products has received much attention in the Scientific and popular press. A considerable amount of this concern is directly related to the effect of trace metals on plants and animals. Although a variety of biochemical and developmental abnormalities due to lead poisoning have been documented in animal systems little information is available concerning the effect of lead on the plant cell. The principal object of the present study is an investigation of the effect of lead on photosynthesis. Following the demonstration (1) that lead salts can inhibit photosynthetic electron transport in isolated chloroplasts at concentrations as low as 50 micron M, we plan: A. To begin a critical analysis of the inhibition of photosynthesis by lead salts, with emphasis on locating the exact site of interference with the electron transport system. B. To study the lead binding reaction at the molecular level. C. To extend our previous work on lead inhibition in higher plants to unicellular algae. Using this less complex system we will ask: 1. To what extent is the lead ion taken into the cell and into the chloroplast? 2. Will the chloroplast level of lead be high enough to impare photosynthesis? D. To study competition of anions (phosphate), cations, and chelating agents (natural and aritifial) with lead ion binding, inhibition and reversal of inhibition. E. To apply our finding to the general study of the organization of photosystem-II and the oxygen envolving mechanism of photosynthesis since lead ion has been the only simple reversible inhibitor of photosystem-II thus far reported. F. Additionally, to extend these studies to the effect of cadium, mercury and other trace metals.